Dense wavelength beam combining (DWBC) techniques spatially superimpose a plurality of input beams to produce a single combined high power output beam. In order to ensure that the combined high power output beam is of sufficiently high quality, i.e. has a sufficiently small beam parameter product (BPP), for desired applications, DWBC techniques provide for wavelength-locking of the individual emitters that emit the plurality of input beams. Wavelength-locking refers to narrowing the emission spectrum of an emitter about a particular wavelength by causing the emitter to emit a substantial majority of its radiation within a narrow wavelength spectrum. DWBC techniques achieve wavelength-locking by providing feedback to each individual emitter that stimulates emission of radiation at wavelengths within the narrow spectrum thereby decreasing the relative population of radiation at undesired wavelengths.
Without wavelength-locking, individual emitters will emit larger portions of radiation at non-desired wavelengths. Radiation at non-desired wavelengths increases the BPP of combined beams produced by spectral-angular dispersive elements, e.g. diffraction gratings. Additionally, radiation having non-desired wavelengths can induce temporal fluctuation in the output power by means of spectral crosstalk between neighboring emitters. Spectral crosstalk refers to the situation where a portion of the radiation emitted by one individual emitter is directed into a different individual emitter as feedback. In order to limit the levels of radiation emitted at non-desired wavelengths and thereby increase the fidelity of the wavelength-locking process, DWBC techniques can utilize wavelength filtering elements. Wavelength filtering elements are designed to remove radiation having non-desired wavelengths from the low power input beams as those beams propagate through external cavities. For example, DWBC techniques and apparatuses including wavelength filtering elements are described in U.S. patent application Ser. No. 14/053,187, U.S. patent application Ser. No. 14/087,985, and U.S. patent application Ser. No. 14/521,487, all of which are incorporated herein by reference.